I. Field of the Invention
The present invention relates to dilatation balloon catheters of the type employed in percutaneous transluminal angioplasty procedures, and more particularly to a method of molding such balloons to reduce their cone stiffness and thereby improve the maneuverability in smaller and more tortious passages of the vascular system.
II. Discussion of the Prior Art
Dilatation balloon catheters are well known for their utility in treating the build-up of plaque and other occlusions in blood vessels. Typically, a catheter is used to carry a dilatation balloon to a treatment site, where fluid under pressure is supplied to the balloon, to expand the balloon against a stenotic lesion.
The dilatation balloon is affixed to an elongated flexible tubular catheter proximate its distal end region. When the balloon is expanded, its working length, i.e., its medial section, exhibits a diameter substantially larger than that of the catheter body on which it is mounted. The proximal and distal shafts or stems of the balloon have diameters substantially equal to the diameter of the catheter body. Proximal and distal tapered sections, referred to herein as xe2x80x9cconesxe2x80x9d, join the medial section to the proximal and distal shafts, respectively. Each cone diverges in the direction toward the medial section. Fusion bonds between the proximal and distal balloon shafts and the catheter form a fluid-tight seal to facilitate dilation of the balloon when a fluid under pressure is introduced into it, via an inflation port formed through the wall of the catheter and in fluid communication with the inflation lumen of the catheter.
Along with body tissue compatibility, primary attributes considered in the design and fabrication of dilation balloons are their strength and pliability. A higher hoop strength or burst pressure reduces the risk of accidental rupture of the balloon during dilation. Pliability refers to formability into different shapes, rather than elasticity. In particular, when delivered by the catheter, the dilatation balloon is evacuated, flattened and generally wrapped circumferentially about the catheter in its distal region. Thin, pliable dilatation balloon walls facilitate a tighter wrap that minimizes the combined diameter of the catheter and the balloon during delivery. Furthermore, pliable balloon walls enhance the catheter xe2x80x9ctrackabilityxe2x80x9d in the distal region, i.e., the ability of the catheter to bend in conforming to the curvature in vascular passages through which it must be routed in reaching a particular treatment site.
One method of forming strong, pliable dilatation balloons of polyethylene terrathalate (PET) is disclosed in U.S. Pat. No. RE. 33,561 (Levy). A tubular parison of PET is heated at least to its second order transition temperature, then drawn to at least triple its original length to axially orient the tubing. The axially expanded tubing is then radially expanded within a heated mold to a diameter about triple the original diameter of the tubing. The form of the mold defines the aforementioned medial section, shafts and cones, and the resulting balloon has a burst pressure greater than 200 psi.
Such balloons generally have a gradient in wall thickness along the cones. In particular, larger dilatation balloons, e.g., 3.0-4.0 mm diameter (expanded) tend to have a wall thickness in the working length in the range of from 0.010 to 0.020 mm. Near the transition of the cones with the working length or medial section, the cones have approximately the same wall thickness. However, the wall thickness diverges in the direction away from the working length, until the wall thickness near the proximal and distal shafts is in the range of 0.025 to 0.040 mm near the associated shaft or stem.
The increased wall thickness near the stems does not contribute to balloon hoop strength, which is determined by the wall thickness along the balloon medial region. Thicker walls near the stems are found to reduce maneuverability of the balloon and catheter through a tortious path. Moreover, the dilatation balloon cannot be as tightly wrapped about the catheter shaft, meaning its delivery profile is larger and limiting the capacity of the catheter and balloon for treating occlusions in smaller blood vessels.
U.S. Pat. No. 4,963,133 (Noddin) discloses an alternative approach to forming a PET dilation balloon, in which a length of PET tubing comprising the parison is heated locally at opposite ends and subjected to axial drawing to form two xe2x80x9cnecked-downxe2x80x9d portions, which eventually become the opposite ends of the completed balloon. The necked-down tubing is then simultaneously axially drawn and radially expanded with a gas. The degree to which the tubing ends had been necked-down is said to provide control over the ultimate wall thickness along the walls defining the cones. However, it is believed that the use of the Noddin method results in balloons exhibiting a comparatively low burst pressure.
Copending application Ser. No. 08/582,371, filed Jan. 11, 1996, now U.S. Pat. No. 5,733,301, describes a method for reducing cone stiffness by using a laser to ablate and remove polymeric material from the cone areas after the balloon is blown. It is preferable that the desired result be obtained during the balloon molding operations obviating the need for additional post molding operations.
Therefore, it is an object of the present invention to provide a method for stretch blow molding dilatation balloon having a high burst pressure and hoop strength, but with reduced material mass in the balloon cones, thus reducing cone stiffness and improving the trackability, crossing profile, stenosis recross and balloon retrieval, via a guiding catheter.
To achieve these and other objects of the invention, there is provided a method of making dilatation balloons with reduced cone stiffness. The method comprises the steps of first providing a mold having a cavity including a cylindrical center segment defining a working length of a dilatation balloon body where the center segment is of a predetermined diameter. The mold cavity also includes two opposed end segments, each having an arcuate cone shape tapering from the predetermined diameter of the center segment to a smaller desired balloon shaft diameter. The side edges of the mold are dimensioned to be within about 0.05 in. of the termination point of the arcuate cone at the smaller desired balloon shaft diameter.
Next, a tubular polymeric parison of a predetermined diameter and wall thickness is placed with a mold and the parison has the opposed ends thereof extending beyond the side edges of the mold, the opposed ends being clamped in a tensioning fixture. The mold is heated to bring the temperature of the parison near or above the glass transition temperature of the polymeric material comprising the parison. The tensioning fixture is then longitudinally displaced relative to the mold to initially longitudinally stretch the parison by a predetermined amount to introduce a degree of longitudinal orientation and to neck down the tubular parison to a lesser diameter.
Following this initial longitudinal stretch, a second longitudinal stretching operation is initiated and as the tensioning fixture is being moved to achieve a second stretch, a gas is injected into the tubular parison to radially expand the parison to a limit defined by the mold cavity. At this point, the wall thickness in the working length of the balloon and in its cones is a function of the degree of longitudinal and radial stretching as well as the gas pressure applied to effect the radial expansion.
Following inflation of the balloon within the mold, a third longitudinal stretch is performed by further displacing the tensioning fixtures relative to the mold. It is the third stretch within the above-described mold that is found to remove material from the cone area as the tubing is drawn down to a desired size for a catheter shaft. Removal of material from the cone area renders them more pliable than balloons prepared in the same way but not subjected to longitudinal stretching following the radial expansion of the balloon within the mold. The third stretch also creates an increased number of nucleation sites for crystallization to occur.
After the third stretch operation is terminated, the temperature of the mold is increased such that the biaxially oriented balloon reaches its crystallizing temperature for effectively locking the molecular structure in place.
Following crystallization, the mold is cooled below the glass transition temperature of the polymer so that the crystallization structure of the balloon is not lost. Once the mold has sufficiently cooled, it can be opened and the balloon removed.